The present invention relates to oscillators, and more specifically to oscillators controlled in part by a temperature sensing resonator to produce a stable reference frequency. Even more specifically, the present invention relates to oscillators controlled by a software algorithm that uses a temperature sensing resonator passively to produce a very stable reference frequency.
A microcomputer-controlled crystal oscillator (also referred to as an MCXO) is a type of oscillator, also referred to as a frequency generator or a clock, that produces a very stable reference frequency or reference timing under a variety of temperature conditions. As is known in the art, an MCXO employs a strain compensated cut crystal, also referred to as an xe2x80x9cS/C cut crystalxe2x80x9d or a dual mode crystal oscillator to produce the stable reference frequency. A strain compensated cut crystal is described in U.S. Pat. No. 4,872,765 issued Oct. 10, 1989 to Schodowski, entitled DUAL MODE THERMOMETRIC SENSING DEVICE, hereinafter referred to as the ""765 patent, and which is incorporated herein by reference.
The MCXO actively uses the S/C cut crystal to provide temperature compensation of the output of the MCXO, which typically is produced by a voltage controlled crystal oscillator (VCXO) driven by the S/C cut crystal. For example, an S/C cut crystal dual mode resonator simultaneously excites, energizes, or stresses two modes of oscillation such that two frequencies are generated, e.g. a fundamental resonant frequency and an overtone resonant frequency. Advantageously, the relationship between the two generated resonant frequencies provides information about the exact temperature of the S/C cut crystal. As is conventionally done, the beat frequency is used to correct the MCXO output clock frequency for crystal temperature drift.
The MCXO consists of both analog and digital circuits. The analog circuits consist of discrete analog components, plus an S/C cut crystal, and a VCXO. The discrete components are for a dual mode resonator and an electrical network for separating the fundamental and overtone resonant frequencies. The digital circuit consists of both discrete and integrated electronic components.
Attempts have been made to reduce the size, power and cost of the MCXO; however, in such attempts, discrete analog components were still required. For example, discrete inductors are used to separate the fundamental and overtone resonant frequencies of the S/C cut dual mode crystal resonator, which are essential to the temperature compensation of the MCXO. Disadvantageously, these discrete analog components place a limit on the miniaturization of the device, require additional power and increase the cost of the device.
Additionally, in the conventional MCXO design, the S/C cut crystal is used as an active element, i.e. a resonator or a frequency generator that requires power, such that one of the resonant frequencies excited in the S/C cut crystal is directly used to derive the VCXO""s frequency corrections due to temperature changes. The temperature is determined by the relationship between the resonant frequencies of the S/C cut crystal to generate the stable reference frequency.
The present invention advantageously addresses the above and other needs.
The present invention advantageously addresses the needs above as well as other needs by providing a software controlled crystal oscillator including a software algorithm that uses a temperature sensing resonator as a passive element in order to control an oscillator which produces a very stable reference frequency over a wide temperature range.
In one embodiment, the invention can be characterized as a method for digitally controlling a reference frequency of an oscillator comprising the steps of: locking a first signal produced by a first tunable software oscillator on to a first resonant frequency of a temperature sensing resonator; locking a second signal produced by a second tunable software oscillator on to a second resonant frequency of the temperature sensing resonator; estimating a temperature of the temperature sensing resonator using the first signal and the second signal; estimating the first resonant frequency and the second resonant frequency based upon the temperature; and adjusting the first signal to approximate the estimated first resonant frequency.
In another embodiment, the invention can be characterized as a software controlled crystal oscillator including a controllable oscillator generating a reference frequency, a digital processor coupled to the oscillator via a first digital to analog converter for controlling the reference frequency of the oscillator and a temperature sensing resonator coupled to the digital processor via a second digital to analog converter. Also included is an analog to digital converter coupling an output of the temperature sensing resonator to the digital processor. The temperature sensing resonator produces a signal having a plurality of resonant frequencies that are related to each other by temperature, and the digital processor estimates the temperature of the temperature sensing resonator and controls the reference frequency of the oscillator based upon the temperature.
In yet another embodiment, the invention can be characterized as a system for controlling the reference frequency of an oscillator comprising a processor including a program for performing the following steps: locking a first signal produced by a first tunable software oscillator on to a first resonant frequency of a temperature sensing resonator; locking a second signal produced by a second tunable software oscillator on to a second resonant frequency of the temperature sensing resonator; estimating a temperature of the temperature sensing resonator using the first signal and the second signal; estimating the first resonant frequency and the second resonant frequency based upon the temperature; and adjusting the first signal to approximate the estimated first resonant frequency.
In a further embodiment, the invention can be characterized as a digitally implemented system for controlling a reference frequency of an oscillator comprising a first tunable software oscillator producing a signal to be output to a temperature sensing resonator and a first correlator detector coupled to the first tunable software oscillator, the first correlator detector having an output to control the oscillator that produces the reference frequency. A second tunable software oscillator produces another signal to be output to the temperature sensing resonator and a second correlator detector is coupled to the second tunable software oscillator. An input is also included for receiving an (output signal from the temperature sensing resonator, wherein the input is coupled to the first correlator detector and the second correlator detector. And a temperature frequency compensator is coupled to the first tunable software oscillator and the second tunable software oscillator. The temperature frequency compensator estimates the temperature of the temperature sensing resonator and adjusts the frequency of the signal and the other signal based upon the temperature.
In an additional embodiment, the invention can be characterized as a method of using a temperature sensing resonator oscillator as a filter for controlling a reference frequency of an oscillator including the steps of: digitally generating a first clock signal and a second clock signal, wherein the first clock signal and the second clock signal have the same frequency and the second clock signal is related in phase to the first clock signal; driving the temperature sensing resonator with the second clock signal; correlating the first clock signal with an output of the temperature sensing resonator; and generating, in response to the correlating, a signal to be output to the oscillator, wherein the signal controls the reference frequency of the oscillator.